Aquaporin-4 water channel oligomers are associated with the transverse tubules of skeletal myofibers

Abstract

Transverse (T) tubules comprise a tortuous network inside the skeletal myofibers enclosing a distinct osmotic environment. Here we have examined whether the T tubules contain aquaporin type 4 (AQP4) water channels to mediate rapid transmembrane water flow. Separation of T tubular and sarcolemmal membranes by sucrose density gradient centrifugation revealed that two main isoforms of AQP4, namely M23 and M1, were present in both membrane fractions. Compatible with this, expression of fluorescent Venus-AQP4.M23 in rat muscle showed the protein both in the T tubules and at the sarcolemma. Blue-Native polyacrylamide gel electrophoresis showed that higher order oligomers typical to the AQP4 water channel were present in both membrane compartments. Interestingly, α-syntrophin that mediates binding of AQP4 to the sarcolemmal dystrophin glycoprotein complex was also present in the T tubule fraction. Deletion of the syntrophin-binding sequence of AQP4 increased its mobile fraction at the sarcolemma but not in the T tubules. Taken together, our results strongly suggest that both the sarcolemma and the T tubules harbor higher order oligomers of the AQP4 water channel but the interactions with adjacent macromolecules are different.